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Analysis Details of SPIRE Photmeter Beam Profiles

( Bernhard Schulz, October 2014)

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Initial Beam Profile

The beam profile was reconstructed from four fine scan maps with obsids: 1342186522, 1342186523, 1342186524, 1342186525, and two observations of the same sky region 1342255134 and 1342255135 three years later. The Neptune observations followed the small proper motion of the object. The scan pattern had, unlike standard scan map observations, very narrowly spaced scan legs. The maps scanning in spacecraft-y direction consisted of 150 scan legs separated by 3.9", the orthogonally scanning maps comprised of 210 scans at the same separation. There were actually two sets of each map, each starting in opposite directions to test for dependencies on scan direction. The second set of scans in Y-direction actually started mid-way between the previous +Y scan legs (2" offset), leading to an overall finer spacing of legs along the Z-coordinate. The +Z and -Z directions started with almost perfect overlap.Neptune moved about 6.1" between the mid-points of the first and last observation. The observations were special calibration observations that yielded a number of calibration results, for example the positions of all detector pixels, relative flatfields, and the ratio between integral and peak based flatfields, the so-called extended source gains.

The data processing started with standard archival data reduced with HIPE 12.1 and HIPE 11.1. The differences between these versions are irrelevant for products at Level 1. HIPE 13.0.3221 was used for destriping and map-making. We had to resort to an, at the time, very new development version of HIPE to have newly discovered issues with the initial median timeline subtraction of the destriper fixed. Another issue causing cross-like "shadows" was avoided by excluding the central 5' around Neptune from the destriping procedure.

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The reduction steps were as follows:

Correct for SSO proper motion

Assemble all Level 1 timelines (4 observations for Neptune, 2 for Shadow) into one Level1 product.

Find all readouts within 5' radius from Neptune and set the Master flag as well as flag #5, unless the master flag was already set (only Neptune observations).

Run the Destriper.

For all readouts that had been deselected previously, indicated by bit #5, reset master bit and bit #5 (only Neptune observations).

Run Naive Mapper on destriped level 1 data twice, once with pixel size 1" and once with standard pixel size.

HIPE 9.0.1782 and SPIA 1.8 were used. The map reconstruction was performed after median subtraction excluding a radius of 4 arcmin around the source. The maps have 1 arcsec sky bin size.
Pointing for Neptune proper motion was corrected.
The resulting original files are:

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Background Subtraction

Although Neptune and shadow maps appear very consistent in its background features at first sight, a direct subtraction of the maps (see Fig. 3 left) shows a number of artifacts hinting at a small astrometric shift between both maps and a photometric difference. This is particularly obvious when using the maps at standard pixel size benefitting from higher S/N. The shadow map over-subtracts the sources by about 10%. Scaling down the shadow map by 10% gives a better removal, however the best background subtraction was achieved empirically by Gaussian smoothing of the shadow maps with sigmas = [0.10, 0.03, 0.10] for PSW, PMW PLW respectively and shifting the PSW and PMW by 1.8" in the spacecraft X-direction. The smoothing of the shadow map effectively accounts for the smearing of the background galaxies in the Neptune images that were made while the telescope tracked the proper motion of the planet.